1. Field of the Invention
The invention relates to a method for coating substrates for optical components with essentially rotationally symmetric optical coatings and a coating system suitable for carrying out that method.
2. Description of the Related Art
Optical systems that have a plurality of lenses that must be coated with rotationally symmetric optical coatings in order to reduce their reflectance or allow meeting other requirements are employed on projection systems for the microlithographic fabrication of semiconductor devices and other types of microdevices. If necessary, other optical components, such as the imaging mirrors employed on catadioptric or catoptric projection lenses, some of which may have sharply curved surfaces, will also have to be coated. These optical coatings should normally have an accurately controlled, usually as uniform as possible, optical effect over the entire optically utilized portion of the coated surfaces involved.
The effects of coated surfaces in optical trains are largely dependent upon the film-thickness distributions of the coatings applied to those surfaces, i.e., in the case of rotationally symmetric coatings, their radial film-thickness characteristic, which, in the following, will also be referred to as their “film-thickness distribution.” Flawed film-thickness distributions may have adverse effects, such as a falloff in the transmittances of lenses towards their perimeters, particularly in the case of systems that have sharply curved optical surfaces. In the case of field-lens systems, these effects may be worsened by effects due to optical incidence angles. These incidence angles, which are also termed “incidence angles” or “i-angles,” differ at differing radial locations on curved optical surfaces. While axial rays are normally incident on the optical surfaces of lenses at their centers, some of the rays striking their perimeters are incident at very large incidence angles, where incidence angles ranging from 30° to 70° are fairly common in the case of, e.g., projection lenses having high numerical apertures, which may lead to the optical properties of coated lenses being shifted to shorter wavelengths compared to those at their centers. Variations in the refractive indices of coating materials from the centers of coatings out to their perimeters have already been observed, particularly in the case of coatings on sharply curved optical surfaces, which also makes designing coatings difficult.
The aforementioned variation of the refractive index of coatings over their radii is largely due to a decline in the density of the coating material involved from their center out to their perimeter. Here the term “density” refers particularly to the packing density, the optical density and/or the mass density, which are related. The radial decrease in packing density causes other problems, particularly in the case of systems that operate with short-wavelength ultraviolet light. For example, porous coatings absorb more water than smooth coatings having superior packing densities, which may lead to transmission problems at, for example, wavelengths less than about 280 nm, in particular, at wavelengths of 157 nm or less, since UV-light having a wavelength of 157 nm is strongly absorbed by water, and coating-durability problems. Coating-adhesion problems near the perimeters of sharply curved, coated, optical surfaces have been observed.
The suspicion that has been voiced to date is that the observed variations in the refractive indices of coating materials and supple and poorly adhering coating structures near the perimeters of optical surfaces are attributable to the large incidence angles of coating material that occur near the perimeters of optical surfaces. Large incidence angles have also been blamed for scattering losses and for deleterious coating stresses at the perimeters of planar, evaporatively coated, mirrors (cf. U.S. Pat. No. 5,518,518).
Coating systems equipped with planetary-drive systems are frequently employed in order to allow, for example, simultaneously coating several substrates in order to cut coating costs. A planetary-drive system of the type considered here has a primary carrier that may be rotated about a primary rotation axis and is frequently referred to as a “planet carrier” and numerous rotatable substrate carriers, each of which may be rotated with respect to the primary carrier about a respective substrate-carrier rotation axis, and are also termed “planets.” When depositing rotationally symmetric coatings, each substrate is clamped onto a substrate carrier such that the symmetry axis of the coating surface coincides with the primary rotation axis. Although the primary rotation axis and the rotation axes of the substrate carriers are usually aligned parallel to one another, they may also be inclined with respect to one another. The substrate carriers are arranged with respect to a material source, which is usually mounted on the primary rotation axis, such that those locations on a surface of a substrate mounted on a substrate carrier that face the material source and are to be coated will be coatable with coating material from the material source that is incident at incidence angles that may vary widely, particularly if the surfaces to be coated are curved. Here the “incidence angles” or “angles of incidence” for each location where a coating is to be deposited are defined as the angles between the local normal to the coating surface at that location and the direction of incidence of coating material at that same location, and usually vary with time.
Masking or baffling methods are employed in order to alleviate some of the aforementioned problems in the case of planetary-drive systems of this particular type. One example of a masking method is described in the article entitled “Optical Coatings for UV Photolithography Systems” that appeared in SPIE Vol. 2775, pp. 335-365. Here masks inserted into the planetary-drive system, between the material source and the substrates, that serve as shielding masks and have a special, computed, shape that allows intermittently masking off solid angles corresponding to high coating-deposition rates such that they yielded a desired, overall, film-thickness characteristic were employed. These correction masks are usually arranged in the vicinities of substrates. For every surface shape and every desired film-thickness characteristic, there is an optimal mask geometry that may be computed.
Since it has been found that conventional masking methods are not always the ideal choice, particularly in cases involving coating sharply curved optical surfaces intended for use in applications involving wavelengths falling within the deep-ultraviolet spectral range and shorter wavelengths, there is need for further improving these sorts of masking methods.